Cephalopoda
The Cephalopoda comprises a class of molluscs characterized by muscular arms, or tentacles, that project from the head and surround the mouth. Three divisions, or subclasses, are involved, the Ammonoidea which are extinct and the Nautiloidea and Coleoidea which have modern representatives. The Nautiloidea and Ammonoidea have external, chambered shells and are thus ectocochliate. The Nautiloidea have their beginning in the Late Cambrian and after a near extinction diversified into some 8 orders in the Ordovician only to more or less steadily decline there after. The Ammonoidea, which are derived from the nautiloid Orthocerida, have their beginning in the Devonian and after great diversification went extinct at the end of the Cretaceous. Coleoids differ in that their shells, if present, are internal. They include recent squid, octopus, and cuttlefish. Evolution Cephalopods made their first appearance in the Late/Upper Cambrian with the ancestral Nautiloidea and diversified in the following Ordovician Pulsed cephalopod diversification during the Ordovician. Palaeogeography, Palaeoclimatology, Palaeoecology 1 March 2009. http://www.sciencedirect.com/science/article/pii/S0031018208006639 to become diverse and dominant in the Paleozoic and Mesozoic seas. name="Dzik1981">J. Dzik, 1981. Origin of the Cephalopoda. Acta Palaeontologica Polonica 26:2 p161-191 During the Late Cambrian, cephalopods were most common in shallow near-shore environments, but they have been found in deeper water sediments as well.Ed Landing 2009. The Oldest Cephalopods from East Laurentia. Journal of Paleontology Vol 83 Cephalopods were thought to have arisen from within the tryblidiid Monoplacophora.Main features of cephalopod evolution, The Mollusca vol.12. Palaeontology and Neontology of Caphalopods M.R. Clarke and E.R.; Trueman Eds. Academic Press 1988. However genetic studies suggest that they are more basal, forming a sister group to the scaphopoda but otherwise basal to all other major mollusc classes. Giribet et al. Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons. Proceedings of the National Academy of Sciences of the United States of America. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=16675549(Relevance questioned) of Kentucky; an internal mold showing siphuncle and half-filled camerae, both encrusted.]] Cephalopods are thought to have evolved from a Monoplacophoran-like ancestor H. Lemche and K.G. Wingstrand 1959. The Anatomy of Neopilina galatheae Lemche, 1957 (Mollusca, Tryblidiacea). Galathea Rep. Vol 3, with a curved, tapering shell,and to be closely related to gastropods (snails). The development of a siphuncle allowed their shells to become gas-filled (thus buoyant) in order to support them and keep the shells upright while the animal crawled along the sea floor, and separates the true cephalopods from putative ancestors such as Knightoconus, which lacked a siphuncle. Buoyancy (i.e. the ability to float) came later, followed by swimming in the Plectroneocerida and eventually jet propulsion in more derived cephalopods.Kroger B. 2007. Some Lesser Known Features of the Ancient Cephalopod Order Ellesmerocerida (nautiloidea, Cephalopoda). Palaeontology Vol 50, issue 3 pp 565–572 doi 10.1111/j.1475-4983.2007.00644.x However, because chambered shells are found in a range of molluscs - monoplacophora and gastropods as well as cephalopods - a siphuncle is essential to ally a fossil shell conclusively to the cephalopoda. The earliest such shells do not have the muscle scars which would be expected if they truly had a monoplacophoran affinity. The earliest cephalopod order to emerge was the Plectronocerida , which were quite small, their shells slightly curved, the internal chambers closely spaced. Early cephalopods had fine shells which could not cope with the pressures of deep water.which gave rise to the Ellesmerocerida. These were supplemented in the mid Tremadoc by larger typically straight shelled Endocerida with large siphuncles and coiled Tarphycerida with more narrow siphuncles. By the mid Ordovician these orders are joined by the Orthocerids characterized by longer chambers and narrow central siphuncles with thin connecting rings and Lituitids which begin coiled but become straight during growth. The oncocerids also appear during this time; they are restricted to shallow water sediments and have short conchs (shells). The mid Ordovician saw the first cephalopods with septa strong enough to cope with the pressures associated with deeper water, and could inhabit depths greater than 100 -200m. The wide-siphuncled Actinocerida and the Discocerida both emerged during the Darriwilian. The direction of curvature would prove to be crucial to future evolution of various lineages. Endogastric curvature with the hyponomic sinus on the inside curvature prohibited the development of coiled forms, whereas exogastric curvature with the hypononic sinus on the outer curvature allowed for the evolution of coiling. Cephalopods are predators at or near the top of the food chain.Peter Boyle & Paul Rodhouse, 2004. Cephalopods: ecology and fisheries. Blackwell Publishers, Ames,Iowa. http://books.google.com/books?id=4UtCi2B4VnoC The Ammonoidea and Coleoidea, (including extinct Belemnoidea and modern Neocoleoidea), both diverged from the nautiloidea during the middle Paleozoic between 450 and 300 million years ago, although the coleoids may be polyphyletic.Wilbur, Karl M.; Trueman, E.R.; Clarke, M.R., eds. (1985),'' The Mollusca'', 11. Form and Function, New York: Academic Press, ISBN 0-12-728702-7 It is thought that competitive pressure from fish forced the shelled forms into deeper water, which provided an evolutionary pressure towards shell loss and gave rise to the modern coleoids, a change which led to greater metabolic costs associated with the loss of buoyancy, but which allowed them to recolonise shallow waters. However, some of the straight-shelled nautiloids evolved into belemnites, out of which some evolved into squid and cuttlefish. The loss of the shell may also have resulted from evolutionary pressure to increase manoeuvrability, resulting in a more fish-like habit. This pressure may have increased as a result of the increased complexity of fish in the late Palaeozoic, increasing the competitive pressure.Internal shells still exist in many non-shelled living cephalopod groups but most truly shelled cephalopods, such as the ammonites, became extinct at the end of the Cretaceous. The tentacles of ancestral cephalopods developed from the molluscan foot and is thought to have involved five pairs which surrouned the mouth Shigeno et al 2008 Evolution of the cephalopod head complex by assembly of multiple molluscan body parts: Evidence from Nautilus Embryonic Development. Journal of Morphology Vol 269, no 1. pp 1-17. http://www.ncbi.nlm.nih.gov/pubmed/17654542}} The preservation of cephalopod soft parts is not entirely unusual; soft-bodied fossils, especially of coeloids (squid), are relatively widespread in the Jurassic, Kear, A.J., D.E.G Briggs,& D.T. Donovan 1995. Decay and fossilization of non-mineralized tissue in coleoid cephalopods. Palaeontology 38:1 pp104-132. http://palaeontology.palass-pubs.org/pdf/Vol%2038/Pages%20105-131.pdf but phosphatised remains are unknown before this period. Classification , a vampyromorphid from the Lower Callovian 164.7 Ma.]] The classification shown here follows largely from Current Classification of Recent Cephalopoda http://www.mnh.si.edu/cephs/newclass.pdf (May 2001), plus that regarding fossil groups from several sources. The three subclasses are traditional, corresponding to the three orders of cephalopods proposed by Bather.Bather, F.A. 1888b Professor Blake and Shell-Growth in Cephalopoda. Annals and Magazine of Natural History. Series 6, Vol. 1 pp 421–426 Parentheses indicate extinct groups. Alternative perspectives follow. Shevyrev classification Shevyrev (2005) suggested a division into eight subclasses, mostly comprising the more diverse and numerous fossil forms. Shevyrev, A.A. 2005. The Cephalopod Macrosystem: A Historical Review, the Present State of Knowledge, and Unsolved Problems: 1. Major Features and Overall Classification of Cephalopod Mollusks. ''Paleontological Journal V39:6 pp606-614 Shevyrev, A. A. 2006. The cephalopod macrosystem; a historical review, the present state of knowledge, and unsolved problems; 2, Classification of nautiloid cephalopods. Paleontological Journal V40:1 pp46-54 Class Cephalopoda Cuvier 1795 * Subclass Ellesmeroceratoidea Flower 1950 **Order Plectronocerida **Order Protactinocerida **Order Yanhecerida **Order Ellesmerocerida * Subclass Endoceratoidea Teichert, 1933 **Order Endocerida **Order Intejocerida * Subclass Actinoceratoidea Teichert, 1933 ** Order Actinoceratoidea * Subclass Nautiloidea Agassiz, 1847 ** Order Basslerocerida ** Order Tarphycerida ** Order Lituitida ** Order Discosorida ** Order Oncocerida ** Order Nautilida * Subclass Orthoceratoidea Kuhn, 1940 ** Order Orthocerida ** Order Ascocerida ** Order Dissidocerida ** Order Bajkalocerida * Subclass Bactritoidea Shimansky, 1951 * Subclass Ammonoidea Zittel, 1884 * Subclass Coleoidea Bather, 1888Bather, F.A. 1888a. Shell-growth in Cephalopoda (Siphonopoda). Annals and Magazine of Natural History. Series 6, Vol. 1 p 298–310 Classification of Berthold & Engeser Thomas Berthold and Theo Engeser Berthold & Engeser 1987. Phylogenetic analysis and systematization of the Cephalopoda (Mollusca). Verhandlungen Naturwissenschaftlichen Vereins in Hamburg. (NF) Vol 29, pp187=-220 divided the Cephalopoda in to two primary groups (clades) based on the radula, referred to as Palcephalopoda and Neocephapoda. Most nautiloids belong in the Palcephalopoda. The Neocephalopoda includes modern coleoids, belemnoids, ammonoids, and many orthocerid families.Engeser, Theo 1997. Fossil Nautiloidea Page.http://userpage.fu-berlin.de/~palaeont/fossilnautiloidea/fossnautcontent.htm Phylogeny of coeloids The coeloids may represent a polyphyletic group. References Further reading * A comprehensive overview of Paleozoic cephalopods: Barskov et al 2008. Cephalopods in the marine ecosystems of the Paleozoic. Paleontological Journal Vol 42, pp 1167–1284 External links * Biomineralisation in modern and fossil cephalopods *